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The in-plane rotational stiffness of a shell finite element is fictitious. Different shell elements resolve this in different ways. SHELL63 uses a weak spring to ground to avoid having zero stiffness at this degree of freedom. Since you have supported the model at a single node, the only stiffness available to carry the in-plane moment is the weak spring to ground. The reaction force associated with the in-plane rotation doesn't contain this force as it has zero stiffness.
It is not a good practice to support a shell rotationally at a single point as is done in your test, but apparently SHELL181 can handle it better than SHELL63.
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